Why radiation from Japan's damaged reactors poses little risk in Oregon and elsewhere along the West Coast

View full sizeThe Associated PressA technician works on a radiation monitoring device installed this week in Anaheim, Calif.

Correction appended

Within days of nuclear bomb tests by China in the late 1960s, a Portland monitoring station detected surges in atmospheric radiation more than 60 times the level of radiation that reached Portland after the 1986 Chernobyl nuclear power accident.

When public health officials say that the damaged Japanese nuclear reactors pose no health risk to people in Oregon and elsewhere in North America, they draw on decades of unfortunate experience with large scale releases of radioactive elements. They also draw on extensive studies of the health effects of radiation people routinely encounter in modern society.

In Japan, radiation leaking from the damaged reactors poses a grave danger to workers struggling to prevent a total meltdown. Officials reported radiation levels as high as 400 millisieverts per hour. The millisievert is a common unit for measuring the biological risk of exposure to radiation. At that rate, two-and-half hours of unprotected exposure could cause radiation sickness.

About 15 miles away, the highest radiation level reported Thursday was 0.08 millisieverts per hour, with less than 0.015 millisieverts per hour typical at that distance. By staying indoors, people near the reactor can reduce exposure to about one-tenth outdoor levels.

Nobody can say for sure how much radiation from Japan will reach the West Coast. In the much worse Chernobyl accident in 1986, a steam explosion breached the core of an active reactor and ejected glowing-hot nuclear fuel that triggered immense fires and spewed radioactive material high into the atmosphere and downwind. When the plume reached Oregon, public health officials said levels posed no meaningful risk, but as a precaution advised people relying entirely on rain for drinking water to use other sources.

Atmospheric modeling by an Austrian geophysics laboratory on Thursday predicted that the plume of radioactive iodine-131 and cesium-137 from the Fukushima reactors would become diluted to harmless levels before crossing the Pacific Ocean.

RADIATION AROUND USYou may encounter more radiation in day-to-day life than you realize. The chart compares common radiation sources with some of the known levels from nuclear power accidents in Japan and Chernobyl. Radiation exposure level in millisieverts, which measures biological risks of radiation exposure

Spending an hour outdoors within 10 - 20 miles of the damaged Japanese reactors:0.01-0.02

Correction: An earlier version of this post gave an incorrect figure overstating radiation levels 10 to 20 miles from the Japanese reactors. The levels are 10 to 20 microsieverts per hour, which equals 0.01 to 0.02 millisieverts per hour.